This animation shows how the hot, red steam pushes the piston to drive an old steam locomotive. A precisely controlled 'D' type slide valve on top supplies fresh steam to one side of the piston. The same valve lets old, used up steam from the other side of the piston run out the exhaust port. You can even see the steam lose its energy as it pushes the piston, going from bright red to blue as it gives up work.

4-4-0 locomotive graphic from 'The Young Engineer Book Of Supertrains', by Jonathan Rutland, © 1978 Usborne Publishing Ltd., used with attribution.   Drop me a message at or